Future mobile radio systems will be required to improve their spectral efficiency beyond that which currently exists today. This will mean that modulation schemes in which the information is carried in both the transmitted signal amplitude and phase will increasingly become required. In order to conserve spectrum and to coexist with current radio systems the transmitters using these complex modulations will have to be linear. For example adjacent channel interference levels of &gt;-60 dBc in a 25 kHz channelization may be required.